There has been idle chat that it could create miniature black holes that'll destroy the Earth. This will not happen. It probably won't create black holes. But if it does create black holes they'll be moving so fast that they'll leave the Earth. Even if they don't they'll evaporate due to Hawking radiation as they're so small... and in the unlikely event that it does create a black hole that hangs around to destroy the planet - there'll be nobody to contradict me.

Seriously: cosmic rays have collided with the Earth with huge energies since time immemorial - a particle accelerator experiment in the upper atmosphere. We're still here.

As Wil Wheaton said on Twitter: "Stupid people at table next to me are convinced LHC will destroy the world tomorrow. Resisting urge to tell them how stupid they are. "

(P.S. I'm not saying this to boast, only to show that I know what I am talking about: I post as someone with a damned good Physics degree. Irrelevantly, I've visited LEP, the Large Electron Positron Collider and LHC's predecessor. Tragically, I took time out of my first USA trip to go to Fermilab. I've been to RAL in the UK many times.)